Perishables, such as ice-packed chicken, meat, fishery products, fruits and vegetables are typically shipped in boxes or containers formed of corrugated paperboard which has been coated with a water repellant material. Conventional water-repellant coatings include natural and synthetic waxes and various synthetic organic compounds such as ethylene vinyl acetate. Such coatings improve the water resistance and decrease the water vapor transmission rate of the board so that the box is better suited for shipping perishables in a damp environment than an uncoated board. However, once used, such coated shipping boxes are difficult to recycle, since the coating tends to hold the fibers together and makes them difficult to separate. Thus, conventional wax-based coatings cannot be separated to the degree necessary to enable satisfactory repulping of the board and typically contain particles of the coating ("stickies") which stick to the papermaking equipment, wire, presses and dryer cans and make the resulting furnish unsuitable for papermaking.
Since it has generally not been economical to repulp wax-coated board, used shipping boxes and the like made from water-repellant linerboard have typically been dumped into a landfill or incinerated rather than recycled. Not only is this practice wasteful, it is also environmentally undesirable and a contributing factor to the growing solid waste disposal problem.
In addition, conventional water repellant linerboard can be difficult to work with, especially under warm conditions, since conventional water-repellant coatings may become tacky causing stacked or contiguous sheets of coated board to block or stick together. This blocking often persists after the sheets are cooled and detrimentally affects handling of coated linerboard blanks and boxes made of coated linerboard.
Thus it would be desirable to find an alternative for conventional wax coated and known polymeric water-repellant coatings that have comparable water resistance properties but also have the added benefit of repulpability.
U.S. Pat. No. 4,010,307 to Canard et al. discloses a gloss coating composition comprising an aqueous dispersion containing a binder and a pigment. The pigment comprises 70-95% calcium carbonate and 5-30% of a non-film forming synthetic polymer having particle sizes in the range of 0.05 to 0.3 microns.
U.S. Pat. No. 4,265,977 to Kawamura et al. discloses a paper coating composition comprising an aqueous dispersion of styrene-butadiene copolymer binder in combination with conventional pigment materials and dispersants. The coating formulations contain pigments including clay, talc, calcium carbonate, aluminum hydroxide, barium sulfate as well as pigment dispersing agents and antifoaming agents.
U.S. Pat. No. 4,301,210 to Yasuda et al. discloses an aqueous paper coating composition containing pigment and binder, where the binder may comprise styrene-butadiene copolymer and the pigment material may comprise clay, aluminum hydroxide, calcium carbonate, etc.
U.S. Pat. No. 4,980,024 to Payne et al. relates to a low slip linerboard used in corrugated boxes having increased surface friction. The linerboard is coated with a composition consisting essentially of an acrylamide, glycerine and an antiskid aqueous silica sol.
U.S. Pat. No. 3,985,937 to Fife discloses polymer latex emulsions as a laminating adhesive in the production of corrugated board. The compositions include a styrene-butadiene latex emulsion, clay, paraffin wax and a wetting agent. The wax component is preferably a wax emulsion and is present in the range of 0.1 to 4% based on the amount of polymer latex solid in the composition.
U.S. Pat. No. 4,117,199 to Gotoh discloses a repulpable paper having a coating of synthetic rubber latex and a wax. The coating is produced from an aqueous emulsion containing 100 parts of butadiene-methyl methacrylate copolymer latex and 5-100 parts of a wax emulsion (paraffin and polyolefin waxes). The emulsion coating is dried at an elevated temperature the same as or higher than the melting point of the wax.
The art does not provide aqueous pigmented/polymer/wax formulations coated on linerboard substrates to produce a paper product with low slip, water resistent and heat resistent characteristics that is easily recycled. The invention is directed to the provision of such recyclable paper stocks. It would be appreciated that advantage over known compositions would be obtained by providing aqueous coatings including pigments, a carboxylated polymeric binder, a wax emulsion and an ionic crosslinking agent to provide a coated paper product with low slip and easy recyclability.
Accordingly it is an object of the present invention to provide a water-repellant paperboard especially well-suited for use in the construction of corrugated boxes for shipment of perishable foods.
A further object of the present invention is to provide a water-repellant board of the character described which exhibits improved repulpability as compared with conventional wax coated boards.
An additional object of the present invention is to provide a water-repellant paperboard of the character described which exhibits low slip and improved resistance to blocking.
Another object of the present invention is to provide a water-repellant paperboard of the character described which is economical to produce and to recycle.
Still another object of the present invention is to provide a coating formulation useful as an overprint varnish over preprinted substrate surfaces to simultaneously provide scuff and water resistance.
Having regard to the foregoing and other objects and advantages, the present invention is directed to a water repellant paperboard which exhibits improved properties, particularly repulpability. The paperboard sheet is especially well suited for use as linerboard in the manufacture of corrugated boxes for shipping and storing food, particularly frozen foods such as ice-packed chicken, meat, fishery products, fruits and vegetables. The invention is useful in demanding applications which require a combination of high water repellency, high wet strength and durability under wet conditions for use in food containers.